1. Field of the Invention
The present invention relates to an inkjet printing system which forms an image by arranging dots on a printing medium using a printing head for ejecting ink in the form of droplets based on image data. In particular, the invention relates to a dot controlling method for suppressing problems of an image on a printing medium attributable to the fact that an ejected ink droplet is divided into a main droplet and a satellite (sub droplet)
2. Description of the Related Art
As a result of the spread of information processing apparatus such as copying machines, word processors, and computers and also communication apparatus, inkjet printing apparatus are known as one type of output devices for printing images (information) from those apparatus. An inkjet printing apparatus forms an image by applying ink to a printing medium. Such an apparatus therefore employs a printing head formed by integrating and arranging a plurality of printing elements having an ink ejection opening and a liquid channel for supplying ink to the opening, and it is configured to eject ink from each of the printing elements according to a printing signal. Further, many apparatus having a plurality of rows of printing heads as described above to allow color printing are provided.
In an inkjet printing method dots are printed by placing ink in the form of flying droplets on a printing medium such as paper, the ink serving as a printing liquid. Such a system is advantageous in that it produces low noises because it is a non-contact system. An image can be printed in a higher resolution at a higher speed by integrating nozzles for ejecting ink in a higher density. Further, an image of high quality can be provided at a relatively low cost on a printing medium such as plain paper without any special process such as developing and fixing. In particular, on-demand type inkjet printing apparatuses are expected its promising future because such apparatuses can be easily adapted for color printing and can be made compact and simple.
In the field of such an inkjet printing apparatus, in particular, demand for higher speeds and higher quality is recently increasing. Under the circumstance, nozzle integrating and arranging techniques have advanced rapidly, and many long printing heads having a nozzle array in a high density have become available. As the density of nozzles increases, techniques for reducing the amount of ink ejected from each nozzle are pursued. Further, printing apparatus are provided, in which the gradation of an image is improved by adopting a technique for ejecting droplets in a plurality of sizes from a single nozzle and a configuration for providing a plurality of rows of nozzles for each size of droplet ejected. In order to achieve higher speeds, techniques are being developed for achieving a higher ejection frequency in ejecting ink from nozzles and for moving a carriage carrying a printing head at a higher speed accordingly.
When the ejecting condition of each nozzle of an inkjet printing head becomes unstable, an ink droplet ejected by one erecting operation can be divided into a main droplet and a sub droplet (satellite) smaller than the same. Since the main droplet and the satellite fly at different speeds generally, the two droplets ejected while the carriage is moved will be placed in different positions on a printing medium. In the case that the dot formed by the satellite is too noticeable, the image has a problem in that a dot is observed in a position having no relation with the image data. On the contrary, even when such a satellite is generated, if the satellite is sufficiently smaller in quantity than the main droplet or placed in a position that is very close to the dot formed by the main droplet, a resultant image will have no problem in most cases.
Various solutions to such a problem of a satellite have already been proposed, the solutions including, for example, limiting nozzles to be used for back and forth printing scans and printing contours of characters and graphics where the problem or a satellite is remarkable using nozzles different from those used for regions other than the contours. For example, such solutions are disclosed in Japanese Patent Application Laid-open Nos. 06-135126, 2001-129981, 2002-086764, or 2002-144608.
However, the recent advance toward smaller droplets described above has sometimes made the presence of a satellite more significant in deed, although a reduction in the size of main droplets provides the effect of reducing granularity of an image. When the moving speed of a carriage is increased, a main droplet and a satellite flying at different speeds are placed in positions at an increased distance from each other, which has also had the effect of making a satellite noticeable. The presence of such satellites generated in positions unrelated to image data can result in variation of the gradation of an image to make the rendering of densities unstable.
Further, in the case of a printing apparatus capable of ejecting droplets in a plurality of amounts, main droplets and satellites will form dots in various sizes at various distances from each other, and a situation can occur in which dots in various sizes exist in disorder. When the relationship between positions of placement of main droplets and satellite changes for some reason, various problems can occur in an image, including increased granularity of the image as a whole and generation of stripes or irregularities.
Under the recent circumstance where inkjet printing apparatus are required to achieve higher and-higher image quality, the presence of satellites and the influence of the same on an image is becoming again a serious problem that can be ignored.